Thread: Why do we still see old light?

When we are looking at distant starts and galaxies, some are as far as 5 billion light years away. So that means that it takes 5 billion years for light to get to us. But once the light does get to us, how come it I can still see it some time later.
If I look through a telescope and see light from a dead star now, which is maybe 1 billion light years away, does the fact that I can see that light means that it had reached me? If it has already reached me, and the star is dead, why doesn't the light just keeps going past me.
Im not sure if my question is clear, but Im always imagining that once the light source is finished, the light should cease to exist as well. Yet we can continue to look at light coming from dead stars.

Thanks

If a star stopped emitting light, then eventually it would disappear when the last of the light arrived. But all the things we see (and can still) see are still emitting light (or were, when you take into account the tiem it took for the light to get here...)

Take a simple example: we can see the sun everyday because it is continuously emitting light. If it suddenly stopped, then 8 minutes later the last of that light would pass us and it would get very dark and very cold. The same is true for something further away. It just takes longer before we see it change.

If you look at a star 1 billion light years away and it died 999,999,999 years ago then you wouldn't see any light from it. If it died 1,000,000,001 years then you will still see light from it for another year.

But why would I see the light for one year?
If I use our sun as an example. Lets say the sun is 'off', then it comes 'on' then 'off' again. Like a light switch . I would see that light 8 min later, but will
I see that light for 8 minutes? I think I would see it for a brief moment and then it would be gone.
So in your example, why am I seeing the light for 1 year, doesn't the fact that I can see it, means it has already travelled the distance. I should see it for a moment and thats it?

Originally Posted by leok31

But why would I see the light for one year?
If I use our sun as an example. Lets say the sun is 'off', then it comes 'on' then 'off' again. Like a light switch . I would see that light 8 min later, but will
I see that light for 8 minutes? I think I would see it for a brief moment and then it would be gone.
So in your example, why am I seeing the light for 1 year, doesn't the fact that I can see it, means it has already travelled the distance. I should see it for a moment and thats it?

You do see it "for a moment and that's it".
The light you see now left the Sun 8 minutes ¹ ago.
THEN you see the light that left 8 minutes and a bit of a second later, then a bit of a second after that.
The light you saw "now" (i.e. that left 8 minutes ago) passes on past you.
You see light "for a year" because the Sun shines for "a year".

1 That figure is rounded off.

If the sun is off and then turns on for 5 minutes, then goes off again then what you will see, 8 minutes later, is the sun come on for 5 minutes and then go on again. This is no different from a light in your house: you see the light while it is on (after a short delay) you don;t see it when it goes off (after a short delay).

I'm not sure what you are thinking. While the light, sun or star is "on" it continually emits light. Someone a few meters away will see that light a few nanoseconds later (and for as long as the light is on). Someone 1 light year away, will see the light 1 year later (and for as long as the light is on).

Think of the light you see or detect as a steam of particles sent in your direction (this is a gross simplification, but it will illustrate the point). Whatever the light source does - vary in brightness or colour - will change the amount or nature of the 'particles' heading towards you, and you will see this variation in the same sequence over time as it was created, but it will be n seconds later where n is the distance away divided by the speed of the particles per second. Once the particles have left their source, if that source then ceases to emit, it cannot affect the particles which have already left, so you will still see them in the original order of emission.

An even simpler way to look at it would be to think of yourself standing in the middle of the street and your neighbor leaves his house and is walking toward you. just because your neighbor has left his house doesn't mean you don't see him any more. you will continue to see him until he walks past you and then you won't see him anymore.

Now, holding that example in your head think of the light that you are seeing from that star as a wave. you are still seeing the light because the entire wave of the light has not passed you yet. once the wave comes to an end you will no longer see it but right now you are seeing the rest of the wave as if it's your neighbor walking towards you.

imagine that you could travel at exactly C
moving exactly away from a star which you are viewing,
would you see precisely the same star at it's same time forever?
or would it's light seem to go black?

Originally Posted by grmpysmrf

An even simpler way to look at it would be to think of yourself standing in the middle of the street and your neighbor leaves his house and is walking toward you. just because your neighbor has left his house doesn't mean you don't see him any more. you will continue to see him until he walks past you and then you won't see him anymore.

That might be what the OP is thinking and maybe where he is going wrong. It might be better to think of a stream of guests leaving your neighbour's house and walking towards you. You will be passed by guests (photons) continuously. When the house is empty you will still have guests passing you until the last guest passes you.

So if it takes 30 minutes for all the guests to exit the house, you will have guests passing you for 30 minutes.

Similarly, if the star shines for, say, 1 billion years, you will have light from it passing you for 1 billion years.

Originally Posted by sculptor

imagine that you could travel at exactly C
moving exactly away from a star which you are viewing,
would you see precisely the same star at it's same time forever?
or would it's light seem to go black?

As you can't travel at the speed of light, the question is meaningless. However, fast you go, the light from the star will pass you at the speed of light. (It will be red-shifted though.)

Imagine that you are a ball travelling uphill at the speed of light but backwards in time.
Now imagine that you are a penguin flying over the Earth, cutting postage stamps in half with a broken bottle.
Then imagine that you are a tiny ant trying to drive a normal sized car whilst balancing a teapot on top of a magic lasso.

I hope that helps.

Originally Posted by leok31

Im not sure if my question is clear, but Im always imagining that once the light source is finished, the light should cease to exist as well. Yet we can continue to look at light coming from dead stars.

I know physics isn't my strong suit, but I will try to put it in a way that I understand it (If I've gotten any aspects wrongs, I invite others to correct me).

Imagine a lengthy conveyor belt in a postal office. On one end, parcels upon parcels are being loaded onto that belt at point A, a steady stream of parcels still traveling on the belt being point B, and the parcels are being unloaded off the end of that belt being point C. Even if the loading action stops at point A, the parcels that are currently on it (Point B) will still travel along it till it reaches the end of the belt at point C. Since there are parcels still traveling on the conveyor belt (Point B) even after the "loading on" (Point A) has stopped, we will continue to see parcels being fed out of the belt (Point C) until the very last parcel arrives at the end.

I had at one point entertained a different analogy with a vertically placed and extremely long water hose with water still in it but the tap turned off.

I hope this helps.


Edits: I have a quick question related to light source if any one is up for it. "Light" consist of photons, and a star emits light. Does a single photon travel omnidirectionally?

Originally Posted by scoobydoo1

Edits: I have a quick question related to light source if any one is up for it. "Light" consist of photons, and a star emits light. Does a single photon travel omnidirectionally?

No; it travels in a "straight" line. That's why you can't see a flashlight beam unless it's pointed at you. The side view is dark.

Originally Posted by tk421

No; it travels in a "straight" line. That's why you can't see a flashlight beam unless it's pointed at you. The side view is dark.

So, a star that emits light is emitting it (sorta) omnidirectionally, but photons only travel unidirectionally. Thanks.

Originally Posted by scoobydoo1

Originally Posted by tk421

No; it travels in a "straight" line. That's why you can't see a flashlight beam unless it's pointed at you. The side view is dark.

So, a star that emits light is emitting it (sorta) omnidirectionally, but photons only travel unidirectionally. Thanks.

Exactly right. A star spews out photons in all directions, but if you were to follow any one photon, you'd be traversing a straight line path.

Originally Posted by Strange

Originally Posted by grmpysmrf

An even simpler way to look at it would be to think of yourself standing in the middle of the street and your neighbor leaves his house and is walking toward you. just because your neighbor has left his house doesn't mean you don't see him any more. you will continue to see him until he walks past you and then you won't see him anymore.

That might be what the OP is thinking and maybe where he is going wrong. It might be better to think of a stream of guests leaving your neighbour's house and walking towards you. You will be passed by guests (photons) continuously. When the house is empty you will still have guests passing you until the last guest passes you.

So if it takes 30 minutes for all the guests to exit the house, you will have guests passing you for 30 minutes.

Similarly, if the star shines for, say, 1 billion years, you will have light from it passing you for 1 billion years.

Thanks for all that everyone.
So basically what you are saying that just because I can see the light using the telescope, does not mean that it had actually reached me? Because what I am thinking is that if I can see the light, even using the telescope, that means that light has reached me. But you're saying that it is still travelling?

Originally Posted by leok31

So basically what you are saying that just because I can see the light using the telescope, does not mean that it had actually reached me? Because what I am thinking is that if I can see the light, even using the telescope, that means that light has reached me. But you're saying that it is still travelling?

Huh? Obviously, for you to see the light, it must have reached you. The light you see "now" has just reached you and ... now it has passed you. But there is more light immediately behind it (and more light behind that). So you continue to see light, more light as it arrives (and passes you). Until you turn the light off. And then there is no more light flowing past you. So you don't see it any more.

Are you just trying to wind us up?

it's like sitting in the upper deck of the right field bleachers. You hear the crack of the bat as the hitter is heading to first base. But the crack of the bat is heard in full, just a time after the visual preception of the event.

Originally Posted by leok31

So basically what you are saying that just because I can see the light using the telescope, does not mean that it had actually reached me? Because what I am thinking is that if I can see the light, even using the telescope, that means that light has reached me. But you're saying that it is still travelling?

well, the light that you are seeing at any particular moment may already be behind you but it is always replaced by the light that is trailing behind it, until there is no more trailing light and then you will see nothing. so it looks like it's the same light when it's really not.

Originally Posted by Strange

Originally Posted by leok31

So basically what you are saying that just because I can see the light using the telescope, does not mean that it had actually reached me? Because what I am thinking is that if I can see the light, even using the telescope, that means that light has reached me. But you're saying that it is still travelling?

Huh? Obviously, for you to see the light, it must have reached you. The light you see "now" has just reached you and ... now it has passed you. But there is more light immediately behind it (and more light behind that). So you continue to see light, more light as it arrives (and passes you). Until you turn the light off. And then there is no more light flowing past you. So you don't see it any more.

Are you just trying to wind us up?

Sorry Im not,
So the light Im seeing through the telescope coming from a dead star has reached me, but but because it's still emitting light, I am able to see it.

Originally Posted by leok31

So the light Im seeing through the telescope coming from a dead star has reached me, but but because it's still emitting light, I am able to see it.

I'm not sure why you are talking about a dead star. I don't even know what that means. But, yes, the light your seeing through the telescope coming from a star has reached you, but but because it's still emitting light, you are still able to see it.

If by a "dead star" you are talking about a star that is not emitting light, then you won't be able to see it anyway.

Originally Posted by leok31

So the light Im seeing through the telescope coming from a dead star has reached me, but but because it's still emitting light, I am able to see it.

Think of a star's light as consisting of pellets that are shot out continuously. If the star were to stop emitting light for some reason, there would still be pellets in flight that have not yet reached you. What your eye perceives are the strikes of those pellets. Until the pellets in flight have all passed you by, you'll still be seeing the star's light.

As has been mentioned, if our sun suddenly went cold, we wouldn't notice for about 8 minutes, because of all the light-pellets that are still in flight. It would take 8 minutes for those pellets to pass us. After that, no more pellets. And then we'd notice that it suddenly got very dark.

Got it! Thanks

Phew!

Originally Posted by Strange

Phew!

Hey! I did all the work!

If it wasn't for my erudite and enlightening post, he would never have understood.
My post was like an epiphany in text form!
Reading it was like being touched by the hand of god!!



Hmmm....I might have gone a bit too far with the hyperbole.

OK, let's try again. A car factory 100 miles away makes cars, and as soon as they come off the production line they are driven from the factory down the road towards you. You are standing on a bridge over the road and you see a continuous stream of cars passing underneath you. One day the factory closes, but all the cars that have left the factory and are already on the 100 miles of road still continue driving down the road past you. You will continue to see cars passing you until the last one which was completed just before the factory closed passes you. After that one you will see no more cars. The factory is the star, the cars are the light it gives out. Light is a stream of particles or waves travelling towards you. You cannot see it until those particles arrive on the retina of your eyes, or the detector of your telescope.

Red panda, what the hell are you on???

Originally Posted by One beer

Red panda, what the hell are you on???

I'm on the good stuff.

maybe he is from colorado....hahaha

Originally Posted by grmpysmrf

An even simpler way to look at it would be to think of yourself standing in the middle of the street and your neighbor leaves his house and is walking toward you. just because your neighbor has left his house doesn't mean you don't see him any more. you will continue to see him until he walks past you and then you won't see him anymore.

Instead of just your neighbour, imagine a conga line of everybody in your street, passing you one by one.

Just because one person has gone past you doesn't mean there aren't lots of others still to reach your position.

It's like that with light from a distant star. There's a billion years' worth of light currently en route to your location, it's just not reached you yet.

because the universe would be very dark if you could not see old light.

Every bit of light is old light. If you turn a flashlight on 1 foot from your face. the light that left the flashlight still has to travel that foot before it reaches your eyes.

You, however will interpret this as instantaneous for 2 reasons A. because light moves very very fast. Faster than your ability to interpret it as being "old" and B. Nothing moves faster than the speed of light, there is no possible scenario for you to interpret the light arriving before it does

Lets say your arm was 1 light year long and you turned on a very powerful flashlight pointed at you, from the moment you decided to turn the flashlight on the light would arrive instantaneously because the electrical impulses would not be able to travel down your arm faster than the light to your brain to tell you that you did indeed click the button on.

If you was a "mad scientist" and decided to blow up the sun, It would "blow up" instantaneously the moment your sun destroyer reached the sun because nothing would travel faster than light to tell you that it has indeed blown up

If you were in a spacecraft and decided to have your wife run outside to take pictures, your radio transmissions could not out run light to warn her that it was indeed blown up

there is no scenario to exceed light.

Thus, when you look at a star 1 million light years out, if it blew up today we would still see its light for another million years because it would take that long for the last of its light to reach you.

A fun experiment is to bounce a laser off the moon. You, however are not able to see the laser but if you could it would arrive at your eyes at the exact same time it does to your computer.

If you replace the planet on the left with a sun then this diagram shows what happens if a sun was repeatedly switched on and off.



(Obviously, the sun does not emit a tight beam, but it was the best animated gif I could find.)

But this diagram shows the anistropic defrabrication of non-concursive saturated psuedo-photons when they pass through a rotating homogeneous (but polarised) Agnieszka field on a Tuesday.

Originally Posted by GoldenRatio

Lets say your arm was 1 light year long and you turned on a very powerful flashlight pointed at you, from the moment you decided to turn the flashlight on the light would arrive instantaneously because the electrical impulses would not be able to travel down your arm faster than the light to your brain to tell you that you did indeed click the button on.

If you was a "mad scientist" and decided to blow up the sun, It would "blow up" instantaneously the moment your sun destroyer reached the sun because nothing would travel faster than light to tell you that it has indeed blown up

No.
In the first case you'd wait a year ¹, in the second it would be 8+ minutes from the time of "blowing up" to anyone noticing. By definition.

1 Actually considerably more than twice that: nerve impulses travel relatively slowly compared to light, therefore between deciding and the flashlight being turned on would take well over a year ² and THEN a year between that and the light arriving at your eyes.
2 I can't be arsed to Google for how fast nerve impulses do travel, but I wouldn't be surprised if it were 10 years or even longer.

Originally Posted by Dywyddyr

2 I can't be arsed to Google for how fast nerve impulses do travel, but I wouldn't be surprised if it were 10 years or even longer.

More like 3 million years. Even the large trunk nerve impulses travel at only 100 meters/sec

Originally Posted by Janus

More like 3 million years.

Well that's longer than 10 years.
And also f*cks up the experiment: by the time the nerve impulse reached your hand the battery would be dead...

Originally Posted by Dywyddyr

Originally Posted by GoldenRatio

Lets say your arm was 1 light year long and you turned on a very powerful flashlight pointed at you, from the moment you decided to turn the flashlight on the light would arrive instantaneously because the electrical impulses would not be able to travel down your arm faster than the light to your brain to tell you that you did indeed click the button on.

If you was a "mad scientist" and decided to blow up the sun, It would "blow up" instantaneously the moment your sun destroyer reached the sun because nothing would travel faster than light to tell you that it has indeed blown up

No.
In the first case you'd wait a year ¹, in the second it would be 8+ minutes from the time of "blowing up" to anyone noticing. By definition.

1 Actually considerably more than twice that: nerve impulses travel relatively slowly compared to light, therefore between deciding and the flashlight being turned on would take well over a year ² and THEN a year between that and the light arriving at your eyes.
2 I can't be arsed to Google for how fast nerve impulses do travel, but I wouldn't be surprised if it were 10 years or even longer.

I was simplifying it to try and better show the principals of light, where nerve impulses could travel at the speed of light. but if you wanted to get technical it would be a 2 year wait. 1 year for the pulse (traveling at the speed of light for simplification) to reach from your brain to the on button, then another year for the light to come on.

However, the signal from your hand that told you that you clicked the on button would not reach your brain before the light does. no need to get technical on the speed of nerve pulses when you have an arm 10 trillion kilometers long

now onto the sun destroyer. Lets say you were staring at the sun through a telescope, and you witnessed the impact that was able to instantaneously switch the sun off. it wouldnt be an 8 minute wait because what you just witnessed actually happened 8 minutes ago. It would appear instantaneous even though in reality it happened 8 minutes ago

also ducky you knew exactly what i was trying to convey on the property of light. dont get your feathers ruffled

Just a side note:
Light moves at... wait for it... the speed of light!
So in the photon's reference frame, time stands still.
I guess we could say that photons don't age then? Hence, photons have virtually an infinite lifetime?
So even if there were some strange connection with the object that emitted the light, if the photon doesn't experience the passage of time, it would continue on its journey ignorant of the condition of the object that emitted it?
Just speculating....

Originally Posted by william

if the photon doesn't experience the passage of time, it would continue on its journey ignorant of the condition of the object that emitted it?

I say, Yes, but only because time doesn't really exist. it's an arbitrary system that we made up in order to organize things.

Time does exist, the arbitary system we made up was to define it. Hours, days, years ect.

Originally Posted by grmpysmrf

because time doesn't really exist

Not supported by the evidence.

it's an arbitrary system that we made up in order to organize things.

Also not supported by the evidence.

If i asked you to meet me on the west cost. You have 1 dimension. If i told you upper east cost, seattle washington. at the space needle for example. You now have 2 dimensions, but at the bottom or the top. This is our 3rd dimension so I ask you to meet me at the top of the space needle. We now have our XYZ axis to pinpoint where to meet. But when? do we meet tomorrow? next week? 10 years from now? This is our 4th dimension, time.

I got nothing except crappy philosophy that says
"how does time exist if know one is around to define it?"
but that's true of anything.

so... damn.

Originally Posted by Dywyddyr

Originally Posted by grmpysmrf

because time doesn't really exist

Not supported by the evidence.

it's an arbitrary system that we made up in order to organize things.

Also not supported by the evidence.

You get a Like only cause golden ratio was so ridiculously articulate in lending credibility to your vagueness regarding my ignorance.

Originally Posted by grmpysmrf

You get a Like only cause golden ratio was so ridiculously articulate in lending credibility to your vagueness regarding my ignorance.

If YOU had bothered to provide anything other than unsupported statements then I would have reciprocated.

Originally Posted by grmpysmrf

You get a Like only cause golden ratio was so ridiculously articulate in lending credibility to your vagueness regarding my ignorance.

Either way...It's what I do

Originally Posted by Dywyddyr

Originally Posted by grmpysmrf

You get a Like only cause golden ratio was so ridiculously articulate in lending credibility to your vagueness regarding my ignorance.

If YOU had bothered to provide anything other than unsupported statements then I would have reciprocated.

What type of statements could I have provided since you accurately pointed out what I wrote is "not supported by the evidence"?
So, the only way I get an eloquent answer from you is by providing statements that have support, even if no support exist?
you are a genius, duck.

Quack quack

Originally Posted by GoldenRatio

Originally Posted by grmpysmrf

You get a Like only cause golden ratio was so ridiculously articulate in lending credibility to your vagueness regarding my ignorance.

Either way...It's what I do

It most certainly was a compliment

Originally Posted by grmpysmrf

What type of statements could I have provided since you accurately pointed out what I wrote is "not supported by the evidence"?
So, the only way I get an eloquent answer from you is by providing statements that have support, even if no support exist?

You could have given a rationale for why you think this, even if you couldn't give facts.
Or, if you had bothered to even Google the subject - or even take a look at some of the numerous threads on that same topic on this forum - you'd have found that you are wrong: and hence wouldn't have bothered posting your claim in the first place.
Either way you come across as lazy and given to simply spouting the first thing that comes to mind.

you are a genius, duck.

Yes.
So what?

So, I've been thinking about this whole time exists and is a dimension thing since this thread...

So, I'm wondering, based on time existing and being real, that if we can find a way to manipulate it that we should be able to be (visually at least) 2 places at once.
I say visually because obviously you cannot be 2 places at once but if you are in the kitchen at 2:00 and in the living room at 2:01 using the theoretical tools of time (That are as yet to be created) we could hypothetically be able to see 2 instances of ourselves at once. only one would either be in the past or in the future but but visually it would like the same time...

What does anybody think of this?

Originally Posted by grmpysmrf

So, I've been thinking about this whole time exists and is a dimension thing since this thread...

So, I'm wondering, based on time existing and being real, that if we can find a way to manipulate it that we should be able to be (visually at least) 2 places at once.
I say visually because obviously you cannot be 2 places at once but if you are in the kitchen at 2:00 and in the living room at 2:01 using the theoretical tools of time (That are as yet to be created) we could hypothetically be able to see 2 instances of ourselves at once. only one would either be in the past or in the future but but visually it would like the same time...

What does anybody think of this?

I think at best you will run into paradoxes. At worse, you need to explain yourself better as its hard to follow your train of thought sometimes.

Originally Posted by GoldenRatio

I think at best you will run into paradoxes. At worse, you need to explain yourself better as its hard to follow your train of thought sometimes.

I was thinking that if we were able to manipulate time to look as a photograph but in 3d "real time"... basically multiple versions of yourself that aren't really "multiple versions" they are just visual representations of yourself at different times...kinda like a photograph.

I'm still thinking and have come up with possibly portals.


maybe this one will be clearer...


say that from the couch to the fridge is a 30 second walk and if we are able to manipulate time perhaps it would look like portals. so we walk through a portal that takes us a the fridge in a step and that takes us there in the second that it takes to take a step. because we have walked through a time portal.

sorry if it's starting to sound like junk now or it's even harder to understand... I know what I mean I'm just having trouble explaining it.

I'm still wrestling with time as a real thing.

I'll show myself the door.

Originally Posted by grmpysmrf

So, I'm wondering, based on time existing and being real, that if we can find a way to manipulate it that we should be able to be (visually at least) 2 places at once.

Well, there is this: Double Vision! These ‘Twin’ Quasars Are Actually The Same Thing

Originally Posted by grmpysmrf

So, I've been thinking about this whole time exists and is a dimension thing since this thread...

So, I'm wondering, based on time existing and being real, that if we can find a way to manipulate it that we should be able to be (visually at least) 2 places at once.
I say visually because obviously you cannot be 2 places at once but if you are in the kitchen at 2:00 and in the living room at 2:01 using the theoretical tools of time (That are as yet to be created) we could hypothetically be able to see 2 instances of ourselves at once. only one would either be in the past or in the future but but visually it would like the same time...

What does anybody think of this?

~~ I will chime in with a view and hope it offers some help, but will fess up to having not read the whole post.. and I should. We have been down this road a few times.. If we can not see or have not seen a or any event there is two points I make ( there are more.) A unseen event is because it may not have happened., or that it has happened and the light image is racing towards you still. I would argue that time is a part of the location map as much as a point on a grid might be.. This question of old light is negated for my understanding by a simple rule NOTHING about the velocity of light changes because of distance from you. That the great scopes of humanity can 'see' a dim fuzzy light source. We know it to be 13 plus some Billions of light years distant. That light stream of photons visible are all that we know of the source. that most of that light missed planet Earth is understandable.. I will return after I read this whole threads content.

Originally Posted by Strange

Originally Posted by grmpysmrf

So, I'm wondering, based on time existing and being real, that if we can find a way to manipulate it that we should be able to be (visually at least) 2 places at once.

Well, there is this: Double Vision! These ‘Twin’ Quasars Are Actually The Same Thing

Very cool.

~ That the light path has been distorted as such is a well known and understood part of astronomical science I know it as 'gravitational lens-ing'. That light path of photons has been bent and distorted by a massive gravity object very near to that light path to us. To say we are seeing something twice is a invitation of misunderstanding. The question of visible old light is relative to position and light intensity. We see a photon stream as it reaches us.. Not only are some of the objects observed no longer where we see them but, they may no longer exist. Understanding that what we see and observe is the photon stream that has reached us. The Sun light you see is already over eight minutes old. All light is aged by it's travel time to us.